1. Field of the Invention
The invention relates to image processing systems. More particularly, the invention relates to a method for accurately transforming color information between two color spaces having differing dimensions, e.g., between a red-green-blue (RGB) color space and a cyan-magenta-yellow-black (CMYK) color space and vice versa.
2. Description of the Background Art
In printing, image retouching and image processing, it is often necessary to convert colors from one representation (color space) into another. Many computer video monitors and scanners, for example, use red-green-blue (RGB) representations for colors, while printers typically represent colors in terms of the amounts of a variety of differently colored inks (for example, cyan-magenta-yellow-black (CMYK)). As such, in a typical computer system, the RGB color space used to produce an image upon a computer screen must be converted into a CMYK color space to facilitate printing of the image depicted on the screen. However, for any particular two color spaces, it is in many instances much easier to convert in one direction than the other, e.g., convert from CMYK to RGB. For example, converting from CMYK to RGB is relatively easy because the CMYK space has more dimensions than the RGB space.
Specifically, an important task in photocompositing is to take a set of images in CMYK format, modify them, and output the result in CMYK. Many of the intermediate operations (image modifications) are more easily or effectively accomplished in RGB space, so it is often necessary to convert from CMYK to RGB and then back to CMYK. One problem with such a transformation is that a CMYK color space is a four-dimensional space and an RGB color space is a three-dimensional space, so the transformation from CMYK to RGB, though relatively simple, inherently loses color information. In particular, the color information produced by xe2x80x9cblack generationxe2x80x9d during creation of the CMYK image is lost. Black generation describes an amount of black ink substituted for equal parts of Cyan, Magenta and Yellow for printing purposes. Consequently, it is very important that an image processing system be able to convert from CMYK to RGB and back to CMYK and produce a black component of the CMYK image that closely approximates the black component in the original CMYK image.
Thus, a difficult and widely needed color transformation is the transformation from an RGB color space to a CMYK color space that retains, as closely as possible, the black generation of the original CMYK image. U.S. Pat. No. 4,500,919 discloses a particular method for converting from RGB to CMYK which is called the Ink Correction Model (ICM). The patent mentions that the ICM xe2x80x9c. . . could be implemented in one huge lookup table, but this would be uneconomic and would also give no clue as to how to find the data to be stored in the LUT short of an impossibly large printing test.xe2x80x9d [11:21] Since the time of filing of ""919 patent, the cost of memory has been reduced sufficiently that it is no longer uneconomic to use xe2x80x9cone huge lookup tablexe2x80x9d. Furthermore, the ""919 patent states that, in using a table based transformation, a large printing test must be conducted to facilitate color space transformation calibration. However, such printing tests are time consuming and complicate the transformation process.
Therefore, a need exists in the art for a method that rapidly and accurately transforms images from a first multi-dimensional color space, e.g., RGB, into a second multi-dimensional color space, e.g., CKYK, without using a printing test and which preserves as closely as possible the black generation of an original CMYK image.
The present invention overcomes the disadvantages heretofore associated with the prior art. Specifically, the present invention converts pixel values from one color space to another, e.g., RGB to CMYK, using a table of interpolated values. The values in the table are filled using data derived from sample images which have been previously converted in the other direction, e.g., CMYK to RGB. The invention infers from those sample images enough about the forward transformation to build an inverse transformation in the table.
In order to convert from RGB to CMYK while retaining as closely as possible the black generation of the original CMYK files, the present invention examines the CMYK files and implicitly infers a black generation model. It does this by creating a table in RGB space of the CMYK values found in the files. At the beginning, each sample of the RGB table is initialized with a value determined from a default transformation of RGB into CMYK using any default black generation strategy. The choice of this transformation is not very important because it is highly modified in the following steps performed by the invention. Next, each pixel of each CMYK image used for creation of the table is converted into RGB, and then the appropriate entries in the RGB-space table are modified so that the interpolation of the table entries at the RGB values yields a value as close as possible to the CMYK pixel color. Once the table has been constructed, it may be low-pass filtered (smoothed), so that the values are highly continuous and no visible artifacts can be identified in the conversion. If the CMYK values of the input images are converted to RGB and the resulting RGB values are converted back to CMYK using the table described above, the original CMYK values with their original black generation are reconstructed with high accuracy as long as all the input images used the same black-generation strategy. If several input images are used that were created with different black-generation strategies (different UCR, GCR, and the like), the table is constructed using an average of the different strategies.
A key advantage of the current invention is that the user need not know anything about the black-generation strategy used in the CMYK file. It is inferred automatically by the inventive method. In situations where people are collaborating over long distances and it is impractical to do a large series of printing tests to facilitate optimization of the color space transformation process, the invention has great advantages over the prior art.